Three-level NPC Inverter Research Articles

Direct power control with neutral-point voltage balance of grid-connected three-level NPC inverter

Multi-level power converters are nowadays the most appropriate solution for medium-voltage grid-connected renewable energy sources (RES) such as variable speed wind-turbines and photovoltaic (PV) generators. This paper presents a three-level direct power control (DPC) scheme for a grid-connected NPC inverter with neutral point voltage (NPV) balance. The developed DPC scheme provides the active (P) and reactive (Q) powers control and commonly it preserves the voltages of the two DC-link capacitors at one half of the full DC bus voltage. At each sampling period, the adequate space vector is selected from a predefined switching table designed regarding the effect of the inverter's space vectors on both, P and Q powers, and their ability to achieve the NPV balance. Numerical simulations are carried out, and confirm the effectiveness of the proposed DPC approach in controlling the P and Q powers and minimising the fluctuations of the DC link capacitors voltages.

Direct power control with neutral-point voltage balance of grid-connected three-level NPC inverter

Multi-level power converters are nowadays the most appropriate solution for medium-voltage grid-connected renewable energy sources (RES) such as variable speed wind-turbines and photovoltaic (PV) generators. This paper presents a three-level direct power control (DPC) scheme for a grid-connected NPC inverter with neutral point voltage (NPV) balance. The developed DPC scheme provides the active (P) and reactive (Q) powers control and commonly it preserves the voltages of the two DC-link capacitors at one half of the full DC bus voltage. At each sampling period, the adequate space vector is selected from a predefined switching table designed regarding the effect of the inverter's space vectors on both, P and Q powers, and their ability to achieve the NPV balance. Numerical simulations are carried out, and confirm the effectiveness of the proposed DPC approach in controlling the P and Q powers and minimising the fluctuations of the DC link capacitors voltages.

A novel region selection approach of SVPWM for a three-level NPC inverter used in electric vehicle

This paper confers an investigation of a space vector modulation based control strategy of induction motor using a three-level inverter for electric vehicle application. The proposed controller uses a simple v/f control with a novel SVPWM technique. A new method for region selection of SVPWM for a multilevel inverter is implemented for the closed-loop system. Previously the region selection procedure was not considered by most of the researcher in the literature for developing the PWM algorithm. This approach is based on some algebraic equations. The remarkable point here is that it is identical to all the remaining sectors. Hence calculation complexity reduces by making it a simpler implementation. This method can be applied to any number of levels. The potency of the proposed controller is validated through the MATLAB/SIMULINK environment. The performance of the overall system is inspected through transient and steady-state analysis. The neutral point balance of the 3L NPC inverter is established by adopting proper switching sequences.

Neutral Point Potential Balancing Control for NPC Three-Level Inverter Based on Space Vector Modulation

This paper introduced the topology and space vector modulation strategy of NPC three-level inverter. A method of judging the sector where the reference vector located is proposed. The cause of neutral-point potential unbalancing is analyzed. In order to solve neutral-point voltage balancing problem, a control method fully utilizing redundant voltage vectors is proposed. Simulation results verified the correctness and effectiveness of the proposed method.

Topology and Modulation Scheme for Three-Phase Three-Level Modified Z-Source Neutral-Point-Clamped Inverter

This paper presents the topology and modulation technique of a three-phase three-level modified Z-source neutral-point-clamped (MZS-NPC) inverter, which combines a modified Z-source impedance network and a three-phase three-level NPC inverter. The boost factor of the proposed MZS-NPC inverter is twice as high as the three existing representative topologies combing an impedance network with a three-level NPC inverter. A modulation scheme for the proposed topology, based on a maximum boost control method, is designed to achieve the maximum voltage gain with simple implementation and to balance the dc-link neutral-point voltage. A closed-loop control of the ac load voltage in the fuel-cell or photovoltaic applications based on the proposed inverter is realized, in order to supply a desired voltage to the critical load in islanding mode of a microgrid. The boosting ability and operation validity of the proposed topology and modulation technique are demonstrated with simulation and experimental results.

Simulation and implementation of DPWM algorithm for NPC three-level inverter

The application of discontinuous pulse width modulation (DPWM) in high-power inverters can effectively improve the equivalent switching frequency of the inverter and improve the control performance of the system. In this paper, the DPWM3 algorithm of NPC type three-level inverter is researched in detail from the judgment of the sector where the reference vector is located, the calculation of the action time of the space vector, and the selection of the switching sequence. Finally, simulation and experimental results verify the rationality of the proposed algorithm. It provides a certain reference value for the research of NPC three-level inverter discontinuous pulse width modulation algorithm.

A Simplified Common-Mode Voltage Suppression and Neutral-Point Potential Control for the NPC Three-Level Inverter

Neutral-point potential unbalancing is an inherent disadvantage of NPC three-level inverter. Traditional control methods, e.g. the space vector PWM (SVPWM) considering balance factor or the model predictive control (MPC), usually needs large amounts of calculation and the common-mode voltage usually keeps large due to the zero vectors being involved in the modulation or the roll optimization. Aiming at above problems, this paper proposes a simplified model predictive algorithm based on the characteristics analysis on the unbalancing and common-mode voltage. Firstly, an approximate two-level structure containing 12 vectors is proposed by the reduced-order processing for the three-level structure, which solves the calculation complexity problem. Meanwhile, multi-constraints including switching frequency, neutral-point potential deviation and the common-mode voltage, are constructed within the roll optimization items of the MPC scheme to make common-mode voltage being restricted less than 1/6 DC voltage while the switching frequency is almost constant. Finally, experiments were carried out to verify that this method can realize a neutral-point voltage balance and a low common-mode voltage along with less execution time.

Efficiency‐optimised modulation technique of three‐level NPC inverter

The three-level NPC inverter has many advantages, such as the characteristics of high output power, low device stress, and low harmonic distortion of output waveform. It is widely used in the fields of high-voltage conversion and reactive power compensation. As the most commonly used multi-level inverter, three-level NPC inverter has its own disadvantage which is the NP potential fluctuations. It may lead to poor output performance even damage to the devices. Therefore, the problem of NP potential fluctuation of three-level NPC inverter has been the hotspot of its research. This paper presents an efficiency-optimised dual modulation wave carrier-based PWM (DMWPWM) strategy of three-level NPC inverter. First, the basic principle of traditional DMWPWM is analysed, and the deficiency of traditional DMWPWM modulation technique is described. Then, the efficiency-optimised DMWPWM strategy is deduced. Compared with traditional DMWPWM strategy, the new strategy inherits the advantage of traditional DMWPWM that the NP voltage balance can be maintained in any modulation index and power factor. Moreover, it can reduce the system switching loss which leads to efficiency improvement, and it is simple and easy to implement. Finally, the proposed modulation technique is verified by simulation.

Torque Ripple Reduction and Flux-Droop Minimization of DTC With Improved Interleaving CSFTC of IM Fed by Three-Level NPC Inverter

Direct torque control (DTC) is known for its fast control in AC motor drives due to its simple control structure that directly controls the torque without the need for modulation blocks or frame transformations. However, when used in induction motor (IM) drives it has three main drawbacks: large torque ripples, variable switching frequency, and sector flux-droop at the low-speed region due to the employment of the torque hysteresis controller (THC). Torque ripple minimization can be achieved in DTC drives by replacing the originally proposed two-level inverter with a three-level neutral-point clamped (3L-NPC) inverter. Nevertheless, the switching frequency remains variable and low, which produces an elevated torque ripple and asymmetrical switching signals for the inverter. In addition, sector flux-droop resulting from driving the IM at the low-speed region produces a high current distortion that consequently eliminates the robustness of DTC. To alleviate these problems, an interleaving constant switching frequency torque controller-based DTC (CSFTC-DTC) was implemented. It improves the operation of the IM at the low-speed region by increasing the duty-cycle of the applied active voltage vector and reducing the duration of the applied zero vectors. Although the conventional CSFTC-DTC regulates the stator flux of the IM at the low-speed region and minimizes the total harmonic distortion (THD) of the stator current, it produces a high torque ripple-one of the main disadvantages of classical DTC. In this paper, an interleaving CSFTC-DTC is proposed to subdivide the duty cycle of the applied vectors of the 3L-NPC inverter to limit the influence of the large duty-cycle of the applied vectors on flux-regulation and torque ripples. The simulation and experimental results presented validate the effectiveness of the proposed method over the conventional method.

A Direct Power Control of the Doubly Fed Induction Generator Based on the Three-Level NSVPWM Technique

The paper proposes a direct power control (DPC) scheme for the doubly fed induction generator (DFIG) for variable speed wind-power generation. The machine is connected as a generator. Its rotor is fed by a three-level NPC inverter. We propose to control the DFIG with a technique based on the DPC control performances. A combination of a three-level neural space vector pulse width modulation (3L-NSVPWM) strategy and reactive and active power controllers is made to replace hysteresis comparators used in the traditional DPC drive resulting in a fixed switching frequency of the power converter. The performances obtained by using this control technique are shown under Matlab/Simulink software

Control Methods on Three-phase Power Converters in Photovoltaic Systems

<span lang="EN-US">In this paper, a three-phase load connected to a NPC three-level inverter is presented. To generate gate signals for the multilevel inverter, two commands are developed and compared: the phase disposition pulse width modulation (PDPWM) and the space vector pulse width modulation (SVPWM). DC supply is provided by photovoltaic cells. Boost converter controls the power transfer from photovoltaic generator. Due to nonlinear I-V characteristics of photovoltaic cells, a maximum power point tracking algorithm is adopted to maximize the output power, the nonlinear controller (sliding mode) is developed and simulated. To verify the effectivnesse of the introdueced controller, it is compared with the fuzzy logic controller. Matlab-simulink is used for simulation, analysis and interpretation the results of these controllers.</span>

Improvement of a three-phase converter for combined power supply systems for local objects with a photovoltaic solar battery

The purpose of the work is to improve the principles of forming the output current of a grid inverter to ensure compliance with the standards of parameters of a grid current without increasing the energy losses in the inverter's keys. The application is considered in a multifunctional three-phase converter with round-the-clock use for combined power supply systems of local objects, which provides a power factor close to unity at the point of connection to the AC grid. The methods of the electric circuit theory were used in combination with computer simulation. The analysis of the known principles of forming of an instantaneous value of output current of inverter by the deviation from the referenced value at a constant and variable key switching frequency is performed. For a grid three-level inverter, the use of combined unipolar and bipolar modulation with a relay current controller is suggested. The structure of the current controller is designed to provide the distribution of the unipolar and bipolar modulation intervals in accordance with the referenced value of inverter current and the mains voltage. A mathematical model is developed for studying the processes in the system, which include "a grid - the converter - non-linear unbalanced load", with an estimate of the power losses in the keys of a grid inverter in accordance with their currents and data sheets used, for a different principles of current formation for a two-level and three-level bridge a grid inverter. The results of the simulation confirm the possibility of ensuring the harmonic composition of the grid current corresponding to the standards with the lowest energy losses in the keys in all modes of the converter operation when using a three-level NPC inverter with clamped diodes. Further improvement in performance is possible when using the structure of three-level TNPC inverters.

Fault Detection Methods for Three-Level NPC Inverter Based on DC-Bus Electromagnetic Signatures

This paper proposes two new open-circuit fault detection methods suitable for the diagnosis of power electronics converters. The faulty semiconductor devices are identified using conducted and radiated electromagnetic signatures of the dc bus through nonintrusive measurements. The first method uses a low-cost electromagnetic interference filter to collect the common-mode emissions signature. The second one utilizes an external antenna to collect the emitted near-field signature. Both methods are tested on a three-level neutral point clamped inverter (NPC) inverter with the aim to identify the clamping diodes open-circuit faults. Indeed, each open-circuit fault affects the common-mode emission signature in the time-domain, while the fast Fourier transform of the emitted near-field showed substantial reduction of spectrum amplitude at a specific radio frequency range and the appearance of a new spectral rail. The effectiveness of these two methods has been tested through numerical simulations and validated by experimental results which confirmed its high performance in detecting single as well as multiple open-circuit faults for three-level NPC inverter.

Optimized Low-Switching-Loss PWM and Neutral-Point Balance Control Strategy of Three-Level NPC Inverters

Optimized Low-Switching-Loss PWM and Neutral-Point Balance Control Strategy of Three-Level NPC Inverters

Hybrid Energy Storage System Microgrids Integration for Power Quality Improvement Using Four-Leg Three-Level NPC Inverter and Second-Order Sliding Mode Control

Rising demand for distributed generation based on renewable energy sources (RES) has led to several issues in the operation of utility grids. The microgrid is a promising solution to solve these problems. A dedicated energy storage system could contribute to a better integration of RES into the microgrid by smoothing the renewable resource's intermittency, improving the quality of the injected power and enabling additional services like voltage and frequency regulation. However, due to energy/power technological limitations, it is often necessary to use hybrid energy storage systems (HESS). In this paper, a second-order sliding mode controller is proposed for the power flow control of a HESS, using a four-leg three-level neutral-point-clamped (4-Leg 3L-NPC) inverter as the only interface between the RES/HESS and the microgrid. A 3-D space vector modulation and a sequence-decomposition-based ac-side control allow the inverter to work in unbalanced load conditions while maintaining a balanced ac voltage at the point of common coupling. DC current harmonics caused by unbalanced load and the NPC floating middle point voltage, together with the power division limits, are carefully addressed in this paper. The effectiveness of the proposed technique for the HESS power flow control is compared to a classical PI control scheme and is proven through simulations and experimentally using a 4-Leg 3L-NPC prototype on a test bench.

High Performance Multistring Converter Topology for Three-Phase Grid Tied 200 kW Photovoltaic Generating System

Under various external conditions, grid connected PV system performance is strongly affected by the topology that is used to connect a PV system with grid. This research aims to design a multistring based converter topology for three-phase grid connected 200 kW PV system that has a high performance in various operating conditions. Research was done by a simulation method using Matlab-Simulink with performance being evaluated including the generated power, efficiency, power quality in accordance with grid requirements, as well as the power flow. In the simulation, multistring converter topology was designed using two dc-dc boost multistring converters connected in parallel to a centralized of three-phase three-level NPC inverter with the size of the string being shorter and more parallel strings as well as the maximum voltage of the PV array of 273.5 V close to dc voltage reference of 500 V. Each dc-dc boost multistring converter have individual MPPT controllers. The simulation results showed that this multistring converter topology had a high performance in various operating conditions. This due to more power generated by the NPC inverter (> 190 kW) at the time of high power generation on the STC conditions (1000 W/m2, 25 oC), the lowest efficiency of the total system is 95.08 % and the highest efficiency of the total system is 99.4 %, the quality of the power generated in accordance with the requirements of grid, as well as the inverter put more active power to the grid and less reactive power to the grid. The response of the inverter slightly worse for loads with greater reactive power and unbalanced.

Fault tolerant control of multiphase multilevel motor drives - technical review

The multiphase multilevel motor drives are a promising solution for some high-power and high-reliability applications, since they have multiple power conversion routes, a large amount of redundant voltage vectors, higher equivalent switching frequencies and superior harmonic performance. It has great significance to exploit their remedial control strategies in depth to fully utilize their high fault tolerant capabilities. This paper will present an updated technical review of fault tolerant control schemes for multiphase multilevel motor drives. Based on exemplification of the diode neutral-pointclamping three-level (DNPC-3L) inverters and T-type NPC three-level (TNPC-3L) inverters fed asymmetric six-phase PMSM drives, the study is taken for both phase-leg faults and switch faults. Furthermore, the discussion on multiple-switch faults, short-circuit switch faults and switch and leg hybrid faulty conditions are discussed in this paper.

Novel switching method for single-phase NPC three-level inverter with neutral-point voltage control

ABSTRACTThis paper proposes a novel switching method with the neutral-point voltage control in a single-phase neutral-point-clamped three-level inverter (SP-NPCI) used in photovoltaic systems. A proposed novel switching method for the SP-NPCI improves the efficiency. The main concept is to fix the switching state of one leg. As a result, the switching loss decreases and the total efficiency is improved. In addition, it enables the maximum power-point-tracking operation to be performed by applying the proposed neutral-point voltage control algorithm. This control is implemented by modifying the reference signal. Simulation and experimental results provide verification of the performance of a novel switching method with the neutral-point voltage control.

계통연계형 3-레벨 NPC 인버터의 6차 고조파 제어 기법을 이용한 계통 전류 고조파 저감

계통연계형 3-레벨 NPC 인버터의 6차 고조파 제어 기법을 이용한 계통 전류 고조파 저감

Direct Current Control for Grid-Connected Diode-Clamped Inverters

The accurate control of the line current, robust and dynamic behavior even under distorted grids and other system faults are the most important requirements for grid-connected inverters. Applying direct current control to multilevel inverters combines the advantages of both and is an enabling step towards improved inverter performance. Direct current controllers suffer from high complexity at increased level count, which is resolved by using geometrical principles and simple analytical calculations for switching vector selection within the space vector diagram. The simplified, novel parametric controller concept is scalable to inverters with arbitrary level count, which does not require any switching tables and the new fully symmetric setup guarantees equal switching frequencies among the three phases. The hypotheses are further confirmed on a real hardware test setup on the example of a three-level NPC inverter hardware and a Xilinx development platform. Experimental results prove the expected behavior of the direct current control under various conditions, which shows a general approach to balance the dc-link capacitors of diode-clamped inverters within the theoretical limits and demonstrates the benefits of using field programmable gate arrays as a controller platform.